1. Field of the Invention
The invention relates to a former with a sieve which is constructed as a cylinder mold, so that an adjustable discharge aperture for a fibrous material suspension exiting from a chamber is formed between the sieve and an upper lip formed by a flow guide plate. The upper lip can be adjusted in the direction of the sieve by means of a pressure medium
2. Discussion of Background Information
A former of the foregoing type is known from DE 31 25 378 A1. In this former, a web forming zone is formed, in which the fibrous material suspension, under pressure, passes from a supply channel into an aperture between a cylinder mold and a flow guide plate. The beginning of the aperture is defined between a lower lip and an upper lip, while at the end of the opening, an exit aperture is defined between the sieve and the upper lip. The upper lip in this case is comprised of individual elements that are hinged together. A fixed stop is provided to adjust the exit aperture. The upper lip can be adjusted against the fixed stop with the aid of a spring bellows, which can be acted on hydraulically.
The known arrangement has the disadvantage that the discharge aperture is rigidly defined by the fixed stop and, thus, changes in the web thickness can result from a number of different influential parameters, such as eccentricity of the sieve, changes in the composition of the fibrous material suspension, changes in the feed pressure, etc.
For these reason, in another former marketed under the name "FloatLip Former," the upper lip is acted on aided by a pressure tube, actuated with the aid of a pressure medium, against the sieve. In this manner, relatively uniform support can be achieved through contact of the upper lip, and malformations resulting from deflection/cambering can be prevented. The upper lip can move along with the imperfections of the cylinder mold so that the discharge aperture remains relatively constant.
Nonetheless it has been seen that, even in the known former, variations in the draining pressure resulting from, for example, different degrees of beating the fibrous material suspension or from eccentricities in the sieve still can appear, whereby variations in the thickness of the paper or cardboard web can result. Moreover, the known former does not yet permit a precise, fine adjustment of the draining pressure.
A former with a similar design is known from U.S. Pat. No. 3,645,843. The discharge aperture is delimited by an upper lip that is hinged on the side facing the chamber and is adjustable in the direction of the sieve by means of a pressure medium on the side facing away from the chamber.
Another embodiment of a similar design is known from DE-A-295 08 352. In this embodiment as well, an upper lip of a discharge aperture is adjustable on its end turned from the chamber, in the direction of the sieve by means of a pressure medium.
The embodiments described above all have the disadvantage that variations in the draining pressure can still occur as a result of differing degrees of beating of the fibrous material suspension or eccentricity of the sieve, which can result in variations in the thickness of the paper or board web produced. Moreover, the known formers still do not permit a precise adjustment of the draining pressure.